1. Field of the Invention
The present invention relates to a battery charger and a charge control method capable of discriminating whether the battery to be charged and set in the battery charger is a primary battery or a secondary battery.
2. Description of the Related Art
Nonaqueous secondary batteries like nickel hydride secondary batteries and lithium ion secondary batteries can be restored to the pre-discharged state by charging, and their running cost is lower and more economical than primary batteries such as alkaline cells even when a battery charger and the power consumption upon charging are taken into account.
Until recently, versatile primary batteries have been the main current of power sources of portable electronic devices such as digital cameras having been popularized quickly. However, the main stream is moving to secondary batteries that have been improved in capacity and lifetime in addition to their economical advantage.
Furthermore, since secondary batteries are produced in the same shaped and sizes as primary batteries, electronic devices using them as their power sources need not be changed in battery-accommodating structure too accommodate the secondary batteries, and end-users can freely select and use either primary batteries or secondary batteries.
On the other hand, regarding dedicated battery chargers for secondary batteries, it is presumed that end-users set and charge primary batteries of the same shapes and sizes by mistake, and a design for preventing such accidents has come to be needed.
There various reports on methods of discriminating primary batteries from secondary batteries and methods of stopping the charging when primary batteries are set. Until now, it is the main stream to mount a microcomputer in the battery charger to turn the charging switch on and off.
For example, one of them proposes applying a reference voltage from a battery pack 22 to a monitor resistor 2 via a standard-discriminating resistor 224. In the monitor resistor 214, a monitor voltage determined by the standard of the connected battery 221 is generated. The monitor voltage is converted to a digital value by an A/D (Analog to Digital) converter 215 and sent to a CPU (Central Processing Unit) 212 (see, for example, Japanese Patent Laid-open Publication JP-H07-065864).
However, microcomputers are expensive. Therefore, the method using microcomputers involves the problem of increasing the cost of battery chargers.
Additionally, various battery chargers characterized in low costs are commercially available. However, almost all of them are of a timer charging system that completes the charging after a predetermined length of time. Since they are controlled simply by turning the charging switches on or off, the cost can be very low. However, since they do not have the function of interrupting the charging when a primary battery is set by mistake, they involve the problem of disregarding the safety against error setting of a primary battery.